1. Field of the Invention
The present invention relates generally to methods and apparatus for protecting a switching amplifier from an over-current condition and more particularly to such methods and apparatus that optimize amplifier efficiency.
2. Description of Related Art
In switching amplifiers, such as a Class D amplifier, the input signal is modulated by a pulse width modulator. This modulated signal is applied to the input of an output stage that comprises power FETs. In one application, the output stage is connected to external speakers and drives them according to the modulated signals. One use for such amplifiers is to drive stereo and television speakers.
Because each power FET has a limited current capacity, it can be damaged if excess current flows therethrough. Such excess current can be caused either by a short or by an unintended input signal that can produce excessive current in the power FETs. As a result, it is necessary to protect these power FETs from excess current.
Such protection has to account for excess current produced in several different ways. First, excessive current may be generated as a result of a short of the amplifier output terminals across which the speaker or other load is connected. Second, the output terminals may be shorted to the ground or to the high side of the power supply. Third, an unintended input signal may be applied to the amplifier input and thereby produce excessive current.
In the past, over-current conditions were monitored by driving a second FET and a resistor with the same signal as a power FET. The voltage on one side of the resistor was monitored to determine whether it rose above a pre-defined level. Whenever it did, an over-current protection circuit was triggered.
While this prior art approach provided some protection from over-current conditions, it did so at the expense of amplifier efficiency because the current in the monitoring FET and resistor did not flow through the load, e.g., a speaker.
The present invention will be more fully apparent when the following detailed description is read in view of the accompanying drawings.